Interpretive Summary: Bovine brucellosis causes economic losses due to reproductive failure, and while brucellosis in cattle can be controlled by vaccination, control in wildlife has not been as successful. A study of resistance mechanisms of the causative agent of bovine brucellosis identified differences among the agents in their ability to expel toxic compounds and could relate to differences in infectious cycles of the agents. Identification and characterization of virulence mechanisms have importance regarding devising treatments and for vaccine development.

Technical Abstract:
Classical Brucella spp., which include several zoonotic agents causing livestock losses due to brucellosis, have highly variable susceptibility to erythromycin. As bacterial intrinsic and spontaneous resistance to erythromycin is often traced to 23S rRNA or ribosomal proteins L4 and L22, this suggested differences among Brucella ribosomes. Relative minimum inhibitory concentration (MIC) values of the Brucella reference strains and three marine isolates determined by Etest ranged from 0.032 to >256 µg/ml for the macrolides erythromycin, clarithromycin, and azithromycin and from 2 to >256 µg/ml for the lincosamide, clindamycin. 23S rrn, rplV, tuf-1, and tuf-2 were relatively polymorphic compared to rplD. However, all the EF-Tu1 and EF-Tu2 were identical. Classical Brucella reference and marine strains selected for erythromycin resistance (eryR) had mutations either affecting the ribosome and/or increasing antibiotic efflux. Interestingly, the marine eryR isolates had larger erythromycin MIC value increases than the classical Brucella eryR strains. One mutation occurred in 23S rrn and one in rplV. All other mutations were in rplD, and all of these, except one, occurred among the dolphin or porpoise Brucella eryR mutants. Mutation of the 23S rRNA peptidyl transferase center resulted in resistance to both erythromycin and clindamycin. The presence of the efflux inhibitor L phenylalanine-L arginine beta naphthylamide (PAbN) lowered erythromycin MIC values of reference strains, with one possible exception. MIC values of eryR mutant strains were affected by PAbN, except for B. suis eryR mutants. A phylogenetic tree constructed using concatenated sequences of the ribosomal associated loci formed a clade with three subclusters and B. suis biovar 5.